Barnacles

Darwin had cataloged and described all of the species
collected on his Beagle trip except one: a barnacle.
Darwin felt that he should establish himself as an expert on one
species before daring to make generalizations about all of them.
His growing friendship with Joseph Hooker reinforced this feeling.
Hooker was a botanical expert who, in one of his letters to Darwin,
included a comment which Darwin took to mean that those who speculated
about species and evolution– including himself–should be more careful.
And Darwin himself remained anxious about the consequences of his theory
of evolution by natural selection. Even though he had great confidence
in it, he worried for his reputation and for the survival of his
theory if he published it without being able to support it with irrefutable
evidence. Finally, it turned out that barnacles, far from being
boring, were actually an incredibly strange an interesting species
to study.

Darwin named his barnacle 'Anthrobalanus' to indicate
that it seemed strangely articulated–that is, it had joints. This
fell in line with a discovery that had been made less than twenty
years prior, that the barnacles were actually crustaceans (like
shrimp and crabs) and not mollusks (like clams). Starting with
Anthrobalanus, Darwin conducted a series of microscopic studies
of barnacles that, in the end, would occupy him for eight long
years.

Darwin did not spend eight years on a single species.
As soon as he started studying one species, he realized that he
would have difficulty describing its unique features unless he
had some sense of what the whole family of barnacles was like.
So he started requesting to borrow the samples of other species.
What started as a trickle soon turned into a flood, and Darwin
found himself inundated with offers of specimens from conchologists
and private collectors. He realized that there was no definitive
work on barnacles; in fact, the whole field of barnacle classification
was a mess. Darwin decided that he could be the one to clean up
that mess, so he dived headfirst into a full-scale study of barnacles,
including not only current species but also those that had been
discovered in fossil beds.

Darwin's friendship with Hooker grew. Hooker became Darwin's
main resource regarding evolution, a topic that remained his true
love even while he was in the midst of his barnacle research. Hooker's
botanical expertise was a useful anchor for Darwin's speculations
on the origins of species. Any time he had a question about plant
distribution or varieties, Hooker could be depended on to find
an answer, if there was one. He invited Hooker to spend time at
Down, where the two would go on walks around the grounds before
Darwin, sick as ever, returned to his room to rest.

In January 1847, at the end of one of these week-long
visits, Darwin gave Hooker a copy of the sketch on evolution by
natural selection that he had started in 1842 and expanded in 1844.
It was now 231 manuscript pages long. Hooker's response was what
Darwin was looking for: he was not yet convinced, but he found
it well-reasoned. But his real usefulness was in pointing out the
details that Darwin had overlooked, the places where his argument
could be most easily challenged. Hooker's comments, then and later,
would help Darwin shore up his argument for the day when he was
finally ready to take it to the public.

Over the course of 1847, Darwin's illness, still mysterious,
worsened; he started suffering again from frequent vomiting and
weakness, as he had for years, but now he was also fainting and
seeing spots in front of his eyes. In 1849 he had had enough of
traditional medicine; it had no clue about his illness and did
little to help him. He was skeptical of alternative therapies like
mesmerism, but hydropathy–treating illness with hot, cold, and
high- pressure water–had some appeal. He spent March of 1849 at
a hydropathic spa run by Dr. James Gully. Originally planning to
spend six weeks there with his family in a rented house, he ended
up spending sixteen weeks. The therapy included whole-body treatments
of hot and cold water, showers, vigorous rubbing, and lots of rest.
By the end of it he felt like a new man.